Process for pure grade acrylic acid

ABSTRACT

Processes are provided for producing a pure grade of acrylic acid having residual aldehyde levels under 10 parts per million. The processes apply selected groups of amines sequentially and, preferably, continuously in selectively reducing, for example, acrolein and furfural, and may be advantageously conducted in the presence of maleic acid and maleic anhydride impurities.

This is a continuation-in-part of application Ser. No. 08/251,762, filed May 31, 1994, U.S. Pat. No. 5,571,386, granted Nov. 5, 1996.

This invention relates to a process for purifying acrylic acid. More specifically, the process is a continuous process which provides a pure grade acrylic acid containing very low levels of residual aldehyde.

In the manufacture of acrylic acid by catalytic oxidation of propylene, acrylic acid ("AA") and oxidation byproducts initially are obtained in aqueous AA solution before further purification. In an "extraction/distillation procedure," the aqueous AA solution is extracted with a suitable organic solvent yielding an AA extract which then is azeotropically distilled and dehydrated, thus removing water from the extract and recovering the organic solvent for recycle. In another purifying procedure, a "direct distillation procedure," the aqueous solution extraction step is bypassed and the azeotropic distillation and dehydration applied directly to the aqueous AA solution. By either procedure, the resulting dehydrated AA, or "crude" AA, contains acid impurities, such as acetic acid, maleic acid and maleic anhydride, and also contains aldehyde impurities, such as acrolein, furfural, and benzaldehyde, and other oxidation byproducts. Acetic acid may be removed from the dehydrated AA by fractional distillation to give a low acetic acid AA, also designated "crude" AA, still containing the other components. In the extraction procedure, some of the maleic acid and other acid impurities are rejected to the waste water and thus contribute to expensive waste treatment. In the direct distillation procedure, these same impurities become a waste organic oil which can be burned for its fuel value. Thus, while both processes are practiced commercially, the direct distillation procedure is preferred for newer plants even though its use increases the difficulty in purifying AA containing appreciable levels of maleic acid and maleic anhydride. The present invention can be employed with both processes and is particularly advantageous for the direct distillation process.

Conventional fractional distillation of crude AA ("CAA") is capable of removing most of the maleic acid and maleic anhydride impurities and other high boiling impurities such as terephthalic acid, thus providing a distilled AA that is useful as starting material for producing acrylate esters or some polymers. However, conventional fractional distillation alone is not effective in reducing aldehydes to necessary levels for a pure grade acrylic acid ("PGAA") useful for producing polymers having average molecular weights higher than those of polymers obtained from distilled AA. To obtain PGAA, the CAA from either the extraction/distillation or direct distillation procedure must be purified beyond that achieved by conventional fractional distillation because residual impurities, particularly the aldehydes, interfere with polymerization reactions; aldehyde levels individually must be below about ten parts per million (ppm), more preferably below five ppm, and most preferably below one ppm. PGAA having these aldehyde levels is useful in producing, for example, superabsorbent polymers and polymers efficient as dispersants for oil well drilling muds and as flocculating agents.

It is known that aldehydes may be reduced to ppm levels in AA by distilling AA in the presence of amines or similar compounds. For example, U.S. Research Disclosure no. 167066 discloses that furfural is reduced to <1 ppm by treating crude or distilled acrylic acid with small amounts of phloroglucinol, ortho (o-) phenylenediamine, or aniline; these amines are believed to complex with or decompose furfural to give a product which then may be separated by fractional distillation. U.S. Pat. No. 3,725,208 ("'208") discloses that at least one of the following: sulfuric acid, hydrazine, phenylhydrazine, aniline, mono-ethanolamine, ethylene diamine, or glycine, when added batchwise to partially purified (presumably predistilled) "crude grade" acrylic acid containing aldehydes and the resulting mixture heated for 3 hours at 70° C. prior to fractional distillation, results in an AA distillate containing reduced levels of aldehydes. U.S. Pat. No. 4,828,652 ("'652") teaches that aminoguanidine or its salts are effective when used in ratios of 1 to 3 moles per mole of aldehyde and with at least 1-1.5 hour residence times before fractional distillation of a "technical grade" (again, presumably a distilled grade) of a crude grade AA.

There are problems, however, with the above mentioned aldehyde-reducing methods. For example, known methods utilize so-called single or one-shot amine addition to the crude grades of AA or to a distilled grade of a crude AA, and require appreciable residence time prior to distillation, as disclosed in the '652 patent. That patent also disclosed that previous efforts involving the addition of hydrazine or aqueous hydrazine solutions required an excess of about 4 moles of hydrazine per mole of aldehyde, and special distillation conditions, to achieve a furfural content under 5 ppm; furthermore, under those conditions, the distillation column became coated with byproducts. Although the '208 patent disclosed that amines such as aniline, monoethanolamine and ethylenediamine can be used, under the disclosed conditions the lowest residual aldehyde contents were obtained only with hydrazine or phenylhydrazine and high levels and long residence times were required to achieve low aldehyde level. Additional problems exist when maleic acid and maleic anhydride are present; when their combined level exceeds about 0.1 wt % in CAA, massive solids formation may occur with the excess amine used in either a batch or continuous method. These solids can foul equipment and cause downtime for cleaning. Additionally, because of the competitive reaction of amine with maleic anhydride, excessive quantities of amine need to be added because the reaction with maleic anhydride is kinetically favored over the reaction with furfural and benzaldehyde. Reaction with maleic anhydride may be avoided by predistilling the CAA, but this is a costly step which would beneficially be eliminated. It further has been found that another problem arises if amine is added only to the top or near the top of a fractional distillation column while distilling CAA; amine addition in this manner causes excessive polymer and other solids formation within the column when the CAA contains more than about 10 ppm of acrolein.

Thus, there is need of an efficient process, particularly a continuous process, to produce PGAA from CAA when the CAA contains not only aldehydes but also the typical impurities mentioned above, particularly maleic acid and maleic anhydride. We have discovered an economical, continuous process for reducing aldehydes in CAA containing substantial levels of acrolein, furfural, and maleic acid and anhydride. The process yields a PGAA having less than 10 ppm, and is capable of providing less than 1 ppm, of any individual residual aldehyde without requiring a predistillation of CAA to remove maleic anhydride. Features of the invention also are applicable to earlier processing steps associated with preliminary sources of AA, as will be shown. Further, the inventive process prevents fouling of equipment with polymer and other solids, particularly when certain selected amines are used. The process also provides PGAA with levels of maleic anhydride below 100 ppm and advantageously provides for minimal use of expensive amines and minimal generation of new waste materials as compared to known processes.

Broadly described, the inventive process uses selected groups of amines at different points of the continuous process in providing PGAA. In one embodiment, one or more amine from one selected group of amines (Group A) is added to CAA to provide a crude acrylic acid feed stream. Within the CAA feed stream, the Group A amine rapidly reacts with acrolein and other "light" aldehydes (aldehydes boiling lower than AA), effectively removing them from volatilizing in the column. The CAA feed stream is fed to a fractional distillation column and distilled. The fractionation power of the column retains maleic acid, maleic anhydride and higher boiling components, such as terephthalic acid, near the bottom of the column. Concurrently with the distillation of the CAA feed stream, an amine feed stream of one or more amine from another selected group of amines (Group B) is introduced at or near the top of the column to facilitate removal of any remaining residual volatile aldehydes, particularly furfural, and maleic anhydride. The resulting distillate, which may include polymerization stabilizers, is PGAA.

More specifically, there is provided a continuous process for producing a pure grade acrylic acid (PGAA) comprising the steps of:

a) feeding to a distillation column a crude acrylic acid feed stream at a temperature of from 25° to 100° C., the feed stream comprising:

i) a crude acrylic acid, and

ii) a minimum effective level in a range of from 0.1 to 2.0 molar ratio, based on total moles of aldehydes and maleic acid and maleic anhydride in the crude acrylic acid, of at least one of a Group A amine selected from the group consisting of:

i) a primary arylamine of structure (I), ##STR1## wherein X₁ and X₂ are selected from the group consisting of H, NR₂, OR, Cl, and R, wherein R is selected from H or C₁ -C₆ alkyl;

ii) a hydrazine, optionally its hydrate, of structure R² --NH--NH₂, wherein R² is selected from H, phenyl, 4-nitrophenyl, or 2,4-dinitrophenyl;

iii) an alkylenepolyamine of structure (II) ##STR2## wherein R³ is selected from H or a C₁ -C₆ alkyleneamine, and is a C₁ -C₆ alkyleneamine; and

iv) an α(-amino acid selected from the group consisting of structure III ##STR3## wherein R⁵ is selected from H, R, or R⁴ ; arginine, aspartic acid, glutamic acid, histidine, and methionine;

b) concurrently feeding to an upper portion of the distillation column an amine feed stream comprising a minimum effective level of from 0.01 to 1.0 molar ratio, based on total moles of aldehydes and maleic acid and maleic anhydride in the crude acrylic acid, of at least one of a Group B amine selected from the group consisting of o-, m-, p-phenylenediamine, 4-nitrophenyl-hydrazine, and 2,4-dinitrophenylhydrazine; and

c) fractionally distilling the crude acrylic acid feed stream through the distillation column, in the presence of the amine feed stream, distilling off PGAA having a residual individual aldehyde content less than 10 ppm.

Other amines which have been found effective in the process described immediately above, and which are included in the Group A amine group, include:

i) a primary arylamine of structure (IV), ##STR4## wherein X₃ is selected from the group consisting of --COOH and --COOR⁶, wherein R⁶ is selected from C₁ -C₆ alkyl; and

ii) an alkanolamine of structure (V),

    HO--R.sup.7 --NH.sub.2                                     (V)

wherein R⁷ is selected from C₂ -C₆ alkylene. Their use in the process for producing PGAA is identical to that described immediately above for the previously defined Group A amines.

The continuous process also may be carried out by adding the amine from Group A to a preliminary column used in preparing CAA, e.g., an azeotropic distillation column or an acetic acid removal column, described more fully below. The resulting CAA stream is fed to a final distillation column while concurrently feeding amine from Group B to the upper portion of this final distillation column and distilling off PGAA.

In another embodiment of the invention, another group of amines is particularly useful in reducing maleic anhydride ("maleic") level in the CAA before distillation. The treatment is especially beneficial because it forms maleic adducts which are substantially soluble in CAA, thus preventing pluggage of lines and distillation columns with insoluble materials. This special group of amines is termed Group S, and as with the Group A amines, the Group S amines also are highly effective in reducing acrolein at this treatment stage, before distillation, besides reducing the maleic content of CAA. In this treatment, one or more of a Group S amine is allowed time to react with a CAA stream before the treated stream is further rectified. The holding time in the presence of the Group S amine is determined by the time required to reduce acrolein to less than 10 ppm and maleic anhydride to less than 50% of its initial level in CAA. Following the hold time, one or more amine from another group of amines, designated Group P amines and described below, is added to the Group S-treated stream either immediately prior to distillation or, as previously described for Group B amines, to the upper portion of the distillation column, to further reduce residual aldehydes, particularly furfural and benzaldehyde, to below 10 ppm, during distillation. Thus, there is further provided a process for producing a pure grade acrylic acid (PGAA) comprising the steps of:

a) feeding to a crude acrylic acid feed stream from 0.1 to 2.0 molar ratio, based on total moles of aldehydes and maleic acid and maleic anhydride in the crude acrylic acid feed stream, one or more of a Group S amine selected from the group consisting of:

i) an alkylenepolyamine of structure (II) ##STR5## wherein R³ is selected from H or a C₁ -C₆ alkyleneamine, and R⁴ is a C₁ -C₆ alkyleneamine;

ii) ortho-toluidine, meta-toluidine; and

iii) an alkanolamine of structure (V),

    HO--R.sup.7 --NH.sub.2                                     (V)

wherein R⁷ is selected from C₂ -C₆ alkylene;

b) holding the crude acrylic acid feed stream and the fed Group S amine at a temperature of from 25° to 100° C. for a hold time of from 15 seconds to 150 hours to provide a treated feed stream;

c) feeding the treated feed stream to a distillation column;

d) concurrently feeding from 0.01 to 1.0 molar ratio, based on total moles of aldehydes and maleic acid and maleic anhydride in the crude acrylic acid, of at least one of a Group P amine selected from the group consisting of o-, m-, p-phenylenediamine, 4-nitrophenyl-hydrazine, 2,4-dinitrophenyl-hydrazine, phenyl-hydrazine, hydrazine, and aniline, to the treated feed stream fed to the distillation column, optionally to an upper portion of the distillation column; and

e) distilling the treated feed stream, in the presence of the Group P amine, through the distillation column, thereby providing PGAA having a residual individual aldehyde content less than 10 ppm.

As described previously, CAA is dehydrated AA and typically contains the following acids and aldehydes in the weight amounts indicated: for example, acrolein, ranging from several ppm to about 300 ppm; benzaldehyde and furfural, each about 200-400 ppm establishing a total aldehyde level of from 400 to 1100 ppm; maleic acid and maleic anhydride (combined) up to about 1.0 wt. % (and measured as maleic acid); and other components, such as acetic acid and terephthalic acid. Aqueous AA solutions and the AA extract are AA sources preceding CAA and contain the same acids and aldehydes as the CAA, and water.

In making up the CAA (or other AA source) feed stream, the Group A or S amine may be added to the CAA (or other AA source) either neat or as a solution in a suitable solvent such as water or a saturated carboxylic acid, such as propionic, hexanoic or valeric acid. The Group A or S amine is selected from the amines previously described, and as further defined here. The R-alkyl group of structure (I) of the primary arylamine is a C₁ -C₆ alkyl, that is an alkyl group containing from 1 to 6 carbon atoms in any isomeric form, such as methyl, ethyl, propyl, isopropyl, n-, iso-, or sec-butyl, hexyl, and isomers thereof. R⁶ is a C₁ -C₆ alkyl. Disubstituted primary arylamines, such as diaminotoluenes and dimethyl-anilines, are also effective. The hydrazines (or hydrates) of R² --NH--NH₂ have been defined; the hydrate has the advantage of ease and safety in handling. The C₁ -C₆ alkylene group, R³ and R⁴, of the alkylenepolyamine contains from 1 to 6 carbon atoms, such as methylenic, ethylenic, propylenic, butylenic, hexylenic, and isomers thereof and bears a primary amine. Examples of structure II alkylenepoly-amine include ethylenediamine, diethylenetriamine and dipropylenetriamine. The alpha-amino acid is as described. The C₂ -C₆ alkylene group of R⁷ of alkanolamine V includes, for example, ethylenic, propylenic, butylenic, and hexylenic groups, and isomers thereof. Examples of V include monoethanol amine, and 1,3- and 1,2-propanol amine.

The Group A amine is selected to react rapidly and substantially irreversibly with acrolein and other light aldehydes present in CAA or other AA sources preceding CAA. Most Group A amines react in-line as the CAA or other AA source are being led, with the amine, to a distillation column; provision also can be made to extend residence time by methods known in the art, such as by use of a buffer tank in the continuous feed line. While relatively expensive amines, such as the phenylenediamines, are included in group A, it is preferred to use less expensive amines such as aniline, o-methylaniline, hydrazine hydrate, diethylenetriamine, lysine, methionine, and glycine that are particularly effective with acrolein. (Alkyl amines, such as butyl amine, were found to have lower reaction rates and required higher use levels than, for example, aniline and other "faster-reacting" amines and, accordingly, are not included as Group A amines.) Generally, those Group A amines and their mixtures that are liquids at temperatures below 70° C. are easy to use. For reasons of cost, efficiency, availability, and ease of handling, those amines of Group A preferred for use in the invention include aniline, o-, m-, and p-methylaniline, hydrazine and hydrazine hydrate, diethylenetriamine, glycine, lysine, methionine and monoethanol amine; more preferred, due to their cost and efficiency in reducing acrolein impurity, are aniline, o-methylaniline, hydrazine, hydrazine hydrate and monoethanol amine; aniline and monoethanol amine are most preferred.

At the same time that the CAA feed stream containing the Group A amine is fed to the distillation column, one or more of an amine of Group B is fed, either neat or as a solution as described for the Group A amine addition, to an upper portion of the same column, that is, is fed to the top or within the upper 30% of the column, and always above the Group A-treated CAA feed stream. Effective Group B amines are stated above and are selected to react rapidly and essentially irreversibly with furfural. Preferred Group B amines, due to their cost, availability and efficiency include meta-phenylenediamine, 4-nitrophenylhydrazine and 2,4-dinitrophenyl-hydrazine; of these, meta-phenylenediamine is most preferred. PGAA having residual individual aldehyde level less than 5 ppm is readily achieved using the preferred amines and less than 1 ppm using the most preferred amines.

Another embodiment of the invention utilizing the Group A and B amine groups in similar manner is the addition of one or more amine of Group A to an AA source, for example, to an aqueous AA solution, to form an AA source feed stream which is fed to an azeotropic dehydration column. After dehydration, the resulting CAA has a low acrolein content (<10 ppm) and is dry. The AA source also can be an extract, that is, an aqueous AA solution which has been extracted with a suitable organic solvent; the resulting AA extract is fed to the azeotropic dehydration column in place of the aqueous acrylic acid solution. It is also possible to feed the Group A amine to the azeotropic dehydration column as a separate stream as either AA source is fed to the column. The resulting low acrolein CAA (as now dehydrated) is then fed, optionally, to another fractional distillation column for acetic acid removal, thus yielding a low acrolein CAA having a low (<2,000 ppm) acetic acid content, or directly to a final distillation column. An advantage of adding the Group A amine to an aqueous AA solution or extract is that either solution does not contain maleic acid in its anhydride form, thus not wasting the Group A amine by its reaction with maleic anhydride.

Thus, there is also provided a continuous process for producing a pure grade acrylic acid (PGAA) comprising the steps of:

a) feeding to a first distillation column an acrylic acid source feed stream at a temperature of from 25° to 100° C., the source feed stream comprising i) an acrylic acid source selected from the group consisting of an aqueous acrylic acid solution and an acrylic acid extract; ii) a minimum effective level of from 0.1 to 2.0 molar ratio, based on total moles of aldehydes and maleic acid and maleic anhydride in the acrylic acid source, of one or more of a Group A amine selected from the same group A amines described previously; b) dehydrating the acrylic acid source feed stream to provide a low acrolein crude acrylic acid having an acrolein content less than 10 ppm; c) optionally distilling off acetic acid from the low acrolein crude acrylic acid to provide a low acrolein crude acrylic acid having a reduced acetic acid level; d) subsequently feeding to a final distillation column: i) the low acrolein crude acrylic acid, and ii) concurrently, to an upper portion of the final distillation column, an amine feed stream comprising a minimum effective level of from 0.01 to 1.0 molar ratio, based on total moles of aldehydes and maleic acid and maleic anhydride in the low acrolein crude acrylic acid, of one or more of a Group B amine selected from the group consisting of o-, m-, p-phenylenediamine, 4-nitro-phenylhydrazine, and 2,4-dinitrophenylhydrazine; and e) fractionally distilling the low acrolein crude acrylic acid through the final distillation column, distilling off PGAA having a residual individual aldehyde content less than 10 ppm.

Another embodiment of the invention utilizing, from the amine groups described, the addition of one or more amine of Group A or, in this embodiment, Group S also, to a CAA having a high (>2,000 ppm ) level of acetic acid. This high acetic acid-containing CAA feed stream may be fed to an acetic acid distillation column, that is, a distillation column used for effectively reducing, by its distillation, acetic acid from the CAA. Distillation of the Group A-treated CAA yields a low acrolein (also now low acetic acid) CAA. (The Group A amine can be fed to the acetic acid removal column as a separate stream as the high acetic acid CAA is fed to the column.) Subsequently, the low acrolein CAA is fed to a final distillation column, such as a high purity acrylic acid distillation column, where the final distillation steps are carried out.

Thus, there is additionally provided a continuous process for producing a pure grade acrylic acid (PGAA) comprising the steps of a) feeding to an acetic acid distillation column a feed stream at a temperature of from 25° to 100° C., the feed stream comprising i) a high acetic acid-containing crude acrylic acid; and ii) a minimum effective level of from 0.1 to 2.0 molar ratio, based on total moles of aldehydes and maleic acid and maleic anhydride in the high acetic acid-containing crude acrylic acid, of one or more of a Group A or Group S amine selected from the same group A or S amines described in the preceding descriptions; b) distilling off acetic acid from the feed stream to provide a low acrolein, crude acrylic acid having an acetic acid content less than 2,000 ppm and an acrolein content <10 ppm; c) subsequently feeding to a final distillation column i) the low acrolein crude acrylic acid, and ii) concurrently, to an upper portion of the final distillation column, an amine feed stream comprising a minimum effective level of from 0.01 to 1.0 molar ratio, based on total moles of aldehydes and maleic acid and maleic anhydride in the low acrolein crude acrylic acid, of one or more of a Group B amine selected from the group consisting of o-, m-, p-phenylenediamine, 4-nitrophenyl-hydrazine, and 2,4-dinitrophenylhydrazine; and d) fractionally distilling the low acrolein crude acrylic acid through the final distillation column, distilling off and thereby providing PGAA having a residual individual aldehyde content less than 10 ppm. In this embodiment wherein one of the Group S amines is used, sufficient "hold time" is typically provided during the acetic acid distillation step. Upon feeding the low acetic acid, amine-treated CAA to a distillation column, the Group P amine may be employed by concurrently feeding from 0.01 to 1.0 molar ratio, based on total moles of aldehydes and maleic acid and maleic anhydride in the crude acrylic acid, of at least one of the Group B amine, above, or phenyl-hydrazine, hydrazine, or aniline, to the low acetic acid, amine-treated crude acrylic acid fed to the distillation column, optionally to an upper portion of the distillation column, and then distilling the low acetic acid, amine-treated crude acrylic acid, in the presence of the Group P amine, through the distillation column, thereby providing PGAA having a residual individual aldehyde content less than 10 ppm.

An advantage of these approaches using, for example, either aqueous sources of AA or high acetic acid-containing CAA, is that acrolein and other light aldehydes are removed early in the AA purification process, with the added benefit of reducing the tendency for the AA to polymerize while being purified, thus allowing a reduction in polymerization inhibitor level during further processing. Thus, Group A or S amines can be used effectively in partially purifying acrylic acid in a continuous process step prior to feeding to the final fractional distillation column yielding PGAA and avoiding some of the tendency of acrylic acid to polymerize while being purified. The same preferred Group A and Group B amines described previously are preferred in the aqueous solution sourced AA and high acetic acid CAA embodiments of the invention. PGAA having <10 ppm residual individual aldehyde, preferably <5 ppm, most preferably <1 ppm, may be obtained from the processes described, the preferred amines readily achieving <5 ppm and the more preferred achieving <1 ppm.

Any distillation of the invention is carried out under reduced pressure, typically less than 200 mm Hg, with a column bottom temperature maintained below 150° C., preferably less than about 100° C., to minimize the loss of AA as polymer. When the Group A or S amine is added to the CAA or other AA sources as described, the temperature of the AA-containing feed stream should be greater than 25° C. and preferably greater than 40° C., up to a temperature of 100° C.; a temperature range from 40° to 80° C. is preferred. At these temperatures, fewer solids are generated than at temperatures of 25° C. and below.

The continuous processes of the invention are distinguished from "batch" processes; in the latter, a fixed quantity of CAA or an AA source is charged to a distillation unit, reacted with a fixed amount of amine to reduce aldehyde content (via a single or one-shot amine addition) and subsequently distilled to provide a fixed quantity of purified product. Batch processes, due to the time required for charging, reaction, distillation, and cleanout, characteristically are of lower productivity than the continuous processes of the invention. The inventive continuous processes also are distinguished from "combined batch and continuous" processes where, in the latter, the treatment of CAA or an AA source with an amine to reduce aldehyde content is carried out first in a batch reactor then the pretreated CAA or other source is fed continuously to a distillation column. The latter processes require additional reactors and storage vessels which are unnecessary with the continuous process of this invention. In the embodiment employing Group S amines, amine treatment can be carried out in a batch-type operation, although a continuous mode is preferred. Following Group S treatment, the remaining steps of Group P treatment and distillation are preferably continuous.

The "minimum effective level" of the one or more Group A amine added to the feed of a distillation column (or directly to the column itself) is determined by measuring the acrolein content of the output (e.g. a distillate) resulting from the distillation column or other unit to which the Group A amine-treated stream is fed. (In the embodiments where the Group A amine is added at points in the process prior to the final distillation step, the acrolein content is measured in the major AA-containing stream produced by the unit, e a bottom stream, into which the amine-treated stream is fed.) In any case, the Group A amine is added to the unit's feed source until the measured acrolein level is below 10 ppm. The level of Group A amine required to reduce the acrolein level to <10 ppm is defined as the "minimum effective level" for the Group A amine(s). Group A amine minimum effective levels higher than the minimum effective level usually are needed to achieve lower levels of residual acrolein, such as 5 and 1 ppm, but all Group A amine use levels are within the range specified previously.

The minimum effective level of the Group B amine is determined by measuring the furfural content of the distillate of the final distillation column. Then, increasing levels of the Group B amine are added to the final distillation column until the measured furfural level is <10 ppm. The level of Group B amine required to just reduce the furfural level to <10 ppm is defined as the "minimum effective level" for the Group B amine(s). In the inventive embodiments using Group B amine, the Group B minimum effective level is determined after the minimum effective level for the Group A amine is determined and as Group A amine continues to be fed. Group B amine minimum effective levels higher than the minimum effective level usually are needed to achieve lower levels of residual furfural, such as 5 and 1 ppm, but all Group B amine use levels are within the range specified previously.

The Group S amine is selected both to react rapidly and substantially irreversibly with acrolein and other light aldehydes present in CAA or other AA sources preceding CAA and to form maleic anhydride adducts which are substantially soluble in the treated CAA. The terms "substantially soluble" or "substantially not insoluble" mean that the amount of insoluble solids in the treated CAA mixture is no more than 1 wt. % of the treated mixture weight; it is preferred that the insoluble solids amount is <0.1%. Group S amines are particularly effective when maleic content exceeds 0.1 wt. % of the CAA, where insoluble solids resulting from other, non-Group S amines, may cause in-line solids problems. Most Group S amines react in-line as the CAA or other AA source is being fed, in the presence of the amine, to a distillation column; provision has been made to extend "hold time" by methods known in the art, such as by use of a buffer tank in the continuous feed line. Hold times can be quite short, seconds, when the treated CAA stream temperature approaches, for example, 100° C., and also quite extended, up to 150 hours, when the treated stream temperatures are around ambient temperature. It will be appreciated that holding times could be extended indefinitely, as for example in a batch-type operation, up to weeks; such extended holds are envisioned within the scope of the invention. Normal hold times typically are less than 150 hours.

The minimum effective level of the one or more Group S amine is that which reduces acrolein to less than 10 ppm and maleic anhydride to less than 50% of its initial level within the hold time. Generally speaking, higher levels within the ranges indicated will give shorter hold times. Group S amine minimum effective levels higher than the minimum effective level usually are needed to achieve lower levels of residual acrolein, such as 5 and 1 ppm, and maleic anhydride levels below 50%, but all Group S amine use levels are within the range specified previously.

Group P amines comprise a selected group of amines used for "polishing" the Group S-treated CAA. The criteria for their selection are similar to those described for Group B amines. Indeed, Group P comprises Group B, and also includes the aforementioned aniline, hydrazine and phenyl-hydrazine. Group P amines have been found surprisingly effective when the Group S amine is used for pre-distillation treatment because the Group S amine has effectively reduced maleic and other impurities to sufficiently low level as to allow substantially no insoluble solids formation with the Group P amines before or during distillation.

The resulting PGAA from all embodiments of the invention have residual individual aldehyde contents below 10 ppm.

EXAMPLES General

In the Examples and Comparative Examples these abbreviations are used: CAA, crude acrylic acid; HQ, hydroquinone; MeHQ, monomethyl ether of hydroquinone; mPD, m-phenylenediamine; PTZ, phenothiazine; MEA, monoethanol amine; Ex., Example; r.t., room temperature, meaning ambient or approximately 25° C.; < and > meaning "less than" and "more than", respectively. For compositional analysis, gas chromatography was used for acrolein, benzaldehyde, and furfural and high performance liquid chromatography for maleic acid and anhydride, both methods sensitive to <1 ppm. The maleic acid and anhydride analytical results were combined because the analytical method converted any anhydride into the acid; thus, results for maleic acid and anhydride are reported as "maleic acid/anhydride." (When maleic acid/anhydride is reported for the distillate, it is most likely present as maleic anhydride because it is known from vapor pressure data that negligible maleic acid distills overhead in a column where acrylic acid is distilled overhead.) Where amines were added to CAA or other AA sources, the amount added was expressed as a molar ratio of the amine to the total moles of acrolein, benzaldehyde, furfural, and maleic acid/anhydride measured in the feed stream of the unit to which the feed stream is added. In the case of the distillation examples, the distillate analyses typically were the average of two or more analyses of hourly samples taken during steady, i.e. continuous, operation. Analyses above 100 ppm generally were rounded off to two significant figures.

Screening tests were conducted by adding indicated levels of amine to aliquots of a stock solution of PGAA which had been spiked with the following impurities at the ppm levels indicated in the Examples: acrolein, benzaldehyde, furfural and maleic anhydride. (In the Tables, acrolein, benzaldehyde, furfural and maleic acid/anhydride are denoted by the symbols A, B, F and M, respectively, and results are in ppm.) The aliquots containing the amine were stirred for thirty minutes at 23°-25° C., unless otherwise indicated, and then immediately analyzed. After about five days at 23°-25° C., the aliquots containing amine were reanalyzed. Control samples are those spiked samples into which no amine was added.

The following criteria for the screening tests were used to estimate which amines would be useful as Group A, B, S, or P amines in the invention. An amine was judged to be a useful Group A or S amine if it reduced acrolein to <10 ppm at a molar ratio of amine of up to 2. The amine was judged particularly effective if acrolein reduction occurred within 30 minutes. For an amine to be useful as a Group A or S amine, it did not need to reduce the levels of benzaldehyde and/or furfural to the final low levels of PGAA (accomplished by a subsequent amine from Group B or P). The Group S amine further had to demonstrate substantially no insoluble solids in the CAA stream (i.e. as described previously, no insoluble solids level more than 1 wt. % of the stream weight) and a maleic anhydride reduction to below 70% of the initial level within 5 days at ambient temperature. An amine was judged to be a useful Group B amine if it (i) showed a 50% reduction within 30 minutes in the level of furfural at a molar ratio of amine of less than 1.0, and (ii) showed a reversibility of reaction with furfural (at an amine molar ratio of less than 1.0) which generated, after about 5 days, less than 70% of the initial level of furfural in the control sample. Group P amines had similar criteria: either those of Group B amine or demonstrating at least a 90% reduction of furfural in a distillate when used in conjunction with a Group S amine.

Example 1

Preparation of PGAA where Group A Amine is Aniline and Group B Amine is mPD

A one-inch, fifteen tray Oldershaw column fitted with a steam heated reboiler was used. The CAA, containing 85 ppm acrolein, 220 ppm benzaldehyde, 240 ppm furfural, and 7200 ppm maleic acid/anhydride, was preheated by passing it through a heat exchanger. Aniline was added (0.5 molar ratio) to the flowing preheated CAA and the flowing CAA feed stream was maintained at the desired temperature during feeding to the "pot," i.e., the vessel at the bottom of the column. The operating conditions for the column were: overhead pressure, about 35 mm Hg; CAA rate, about 211 gram/hour; reflux ratio, about 1.6; percent of total feed removed as distillate, about 86%; CAA feed stream temperature, about 50° C.; pot temperature, about 83° C.; and overhead temperature, about 65° C. For polymerization inhibition, the following inhibitor levels were fed based on the CAA rate: about 0.5 wt % air to the reboiler, about 0.1 wt % MeHQ to the condenser, about 0.03 wt % PTZ and about 0.06 wt % HQ to tray 11 (numbered bottom to top). Concurrently, mPD was added (0.08 molar ratio) at tray fifteen, the top of the column. At steady conditions over a period of seven hours, the distillate of PGAA consistently contained <1 ppm each of acrolein, benzaldehyde, and furfural, and 2 ppm of maleic acid/anhydride. There were neither problems with solids formation in the feed lines to the pot or in the pot, nor were there any polymer or other solids problems in the column.

Example 2

Preparation of PGAA where Group A Amine is Hydrazine Hydrate and Group B Amine is mPD

The conditions of Example 1 were repeated except that (i) hydrazine hydrate was fed to the pot (0.5 molar ratio) instead of aniline, (ii) mPD (0.2 molar ratio) was fed to tray eleven, and (iii) the CAA contained 91 ppm acrolein, 210 ppm benzaldehyde, 250 ppm furfural, and 6100 ppm maleic acid/anhydride. At steady conditions over a period of two hours, the distillate of PGAA consistently contained <1 ppm each of acrolein, benzaldehyde, and furfural, and 2 ppm of maleic acid/anhydride. There were neither problems with solids formation in the pot nor were there any polymer or other solids problems in the column. Some minor solids were observed in the CAA feed stream line, but at a level which did not interfere with continuous operation.

Hydrazine was tested without a Group B amine under conditions similar to those described in Example 1, except that (i) hydrazine was fed to the pot at a 0.7 molar ratio and (ii) no Group B amine was fed to the column. At steady conditions over a period of three hours, the distillate contained 2 ppm acrolein, <1 ppm benzaldehyde, <1 ppm furfural, and 13 ppm maleic acid/anhydride. This demonstrated that under certain conditions hydrazine alone (at 0.7 molar ratio) yielded relatively low levels of impurities, but these were not as low as achieved when a Group B amine also was used as in the above Ex. 2.

Comparative Example 1

CAA Distillation without Group A or B Amine Addition

Comparative Example 1 employed conditions similar to those described in Example 1, except that (i) no Group A amine was fed to the pot, (ii) no Group B amine was fed to the column, and (iii) the CAA contained 66 ppm acrolein, 230 ppm benzaldehyde, 270 ppm furfural, and 6600 ppm maleic acid/anhydride. At steady conditions over a period of eighteen hours, the distillate consistently contained 38 ppm acrolein, 3 ppm benzaldehyde, 91 ppm furfural, and 70 ppm of maleic acid/anhydride. There were neither problems with solids formation in the feed lines to the pot or in the pot, nor were there any polymer or other solids problems in the column, but without any amine addition, the levels of impurities in the distillate far exceed those required for PGAA.

Comparative Example 2

CAA Distillation where Group A Amine is Aniline and with No Group B Amine Addition

Comparative Example 2 employed conditions similar to those described in Example 1, except that (i) aniline was fed to the pot (0.6 molar ratio), and (ii) no Group B amine was fed to the column. At steady conditions over a period of two hours, the distillate consistently contained <1 ppm acrolein, 1 ppm benzaldehyde, 46 ppm furfural, and 58 ppm of maleic acid/anhydride. There were neither problems with solids formation in the feed lines to the pot or in the pot, nor were there any polymer or other solids problems in the column. Thus, although aniline fed to the pot sufficiently reduced acrolein, it did not sufficiently reduce furfural to provide a satisfactory PGAA.

Comparative Example 3

CAA Distillation where Group A Amine is Aniline and CAA Feed Stream is at 23°-250° C.

Comparative Example 3 employed conditions similar to those described in Example 1, except that (i) aniline was fed to the pot (0.6 molar ratio), (ii) no Group B amine was fed to the column, (iii) the CAA contained 91 ppm acrolein, 210 ppm benzaldehyde, 250 ppm furfural, and 6100 ppm maleic acid/anhydride, and (iv) the CAA feed stream to the column was maintained at 23°-25° C. Under these conditions, the CAA feed line was plugged with solids after 4 hrs, forcing column shutdown. The solids were identified by ¹ H NMR spectroscopy as N-phenylmaleamic acid, the reaction product of aniline and maleic anhydride.

Comparative Example 4

CAA Distillation Without Group A Amine and where Group B Amine is mPD

Comparative Example 4 employed conditions similar to those described in Example 1, except that (i) no Group A amine was fed to the pot, (ii) mPD was fed to the column (0.05 molar ratio), (iii) the CAA contained 69 ppm acrolein, 230 ppm benzaldehyde, 270 ppm furfural, and 8100 ppm maleic acid/anhydride, and (iv) the CAA feed stream to the column was maintained at 23°-25° C. At these conditions, the column suffered from heavy polymer and other solids formation and after 30 minutes was forced to shut down. Also, the distillate, just prior to shut down, contained 63 ppm acrolein, <1 ppm benzaldehyde, 4 ppm furfural, and 2 ppm maleic acid/anhydride. Thus, feeding amine to the upper portion of the column alone was not satisfactory for PGAA production because of polymer and other solids formation in the column preventing continuous operation. The data showed that heavy polymer and other solids formation occured in the column when no Group A amine was fed to the CAA but a Group B amine was fed to the upper portion of the column.

Comparative Example 5

Screening Tests Adding Primary Alkylamine to PGAA Spiked with Impurities

In Comparative Example 5, the general procedure described previously for screening tests was employed, except that the amines employed were primary alkylamines. The results are given in Table I in which the data show that representative primary alkylamines, n-butylamine and tert-octylamine, were not useful Group A (or Group B) amines. While primary alkylamines may have some utility in reducing acrolein, they would not be effective in producing PGAA.

                                      TABLE I                                      __________________________________________________________________________     Screening Tests Showing the Effect of Primary Alkylamines                      on Aldehyde Removal in Spiked PGAA                                                        Amine                                                                          Molar                                                                              After Thirty Minutes                                                                       After About Five Days                               Additive   Ratio                                                                              A  B  F  M  A  B  F  M                                          __________________________________________________________________________     Control - PGAA Spiked                                                                     0   170                                                                               320                                                                               300                                                                               320                                                                               -- -- -- --                                         n-Butylamine                                                                              0.46                                                                               110                                                                               320                                                                               290                                                                               280                                                                                54                                                                               310                                                                               290                                                                               290                                        n-Butylamine                                                                              2.1  79                                                                               300                                                                               280                                                                               260                                                                                23                                                                               300                                                                               280                                                                               250                                        Control - PGAA Spiked                                                                     0   210                                                                               290                                                                               260                                                                               370                                                                               -- -- -- --                                         Tert-octylamine                                                                           0.47                                                                               190                                                                               280                                                                               270                                                                               360                                                                               110                                                                               270                                                                               240                                                                               340                                        Tert-octylamine                                                                           0.93                                                                               200                                                                               280                                                                               240                                                                               350                                                                                87                                                                               250                                                                               240                                                                               340                                        __________________________________________________________________________

Example 3

Screening Tests Adding Primary Arylamine to PGAA Spiked with Impurities, to CAA, and to Aqueous Acrylic Acid Solution

In Example 3, the general procedure described previously for screening tests was employed, except that (i) the amines employed were primary arylamines and (ii) the amine was added to aliquots of either stock PGAA spiked with impurities, or to CAA, or to aqueous acrylic acid solution. These sources of AA were used containing the ppm levels of acrolein, benzaldehyde, furfural and maleic acid/anhydride indicated in Table II. The aqueous acrylic acid solution contained about 35% water.

Based on the previously described criteria, Table II screening test data show that the following primary arylamines were effective as Group A amines: aniline, m-phenylenediamine, p-phenylenediamine, 1,5-diaminonaphthalene, p-aminophenol, p-methoxyaniline, p-chloroaniline, o-methylaniline, m-methylaniline, p-methylaniline and p-nitroaniline. In addition, Table II shows that m-phenylenediamine is an outstanding Group B amine; p-phenylenediamine also was a useful Group B amine.

                                      TABLE II                                     __________________________________________________________________________     Screening Tests Showing the Effect of Primary Arylamines on                    Aldehyde Removal in Spiked PGAA, CAA or Aqueous Acrylic Acid Solution                     Amine                                                                          Molar                                                                              After Thirty Minutes                                                                       After About Five Days                               Additive   Ratio                                                                              A  B  F  M  A B  F  M                                           __________________________________________________________________________     Control - PGAA Spiked                                                                     0   200                                                                               300                                                                               350                                                                               310                                                                               --                                                                               -- -- --                                          p-Aminophenol                                                                             0.12                                                                               69 300                                                                               370                                                                               200                                                                               30                                                                               320                                                                               400                                                                               310                                         p-Aminophenol                                                                             0.46                                                                               1  290                                                                               330                                                                               30 <1                                                                               290                                                                               360                                                                               290                                         p-Aminophenol                                                                             0.89                                                                               <1 270                                                                               220                                                                               6  <1                                                                               290                                                                               370                                                                               280                                         Control - PGAA Spiked                                                                     0   130                                                                               210                                                                               170                                                                               190                                                                               --                                                                               -- -- --                                          Aniline    0.22                                                                               14 190                                                                               170                                                                               87 <1                                                                               180                                                                               160                                                                               190                                         Aniline    0.46                                                                               <1 150                                                                               120                                                                               25 <1                                                                               170                                                                               150                                                                               210                                         Aniline    0.68                                                                               <1 140                                                                               91 20 <1                                                                               180                                                                               150                                                                               210                                         Aniline    0.93                                                                               <1 120                                                                               79 32 <1                                                                               160                                                                               140                                                                               200                                         Aniline    1.8 <1 88 50 36 <1                                                                               200                                                                               160                                                                               200                                         Control - CAA                                                                             0   150                                                                               250                                                                               260                                                                               7400                                                                              --                                                                               -- -- --                                          Aniline    0.32                                                                               8  260                                                                               270                                                                               4900                                                                              --                                                                               -- -- --                                          Aniline    0.50                                                                               <1 230                                                                               220                                                                               3900                                                                              --                                                                               -- -- --                                          Aniline    1.0 <1 90 20 3400                                                                              --                                                                               -- -- --                                          Control - Aqueous AA                                                                      0   40 130                                                                               110                                                                               4700                                                                              --                                                                               -- -- --                                          Aniline    0.23                                                                               19 140                                                                               120                                                                               4700                                                                               4                                                                               120                                                                               110                                                                               5000                                        Aniline    0.46                                                                               3  120                                                                               94 4600                                                                              <1                                                                               100                                                                                86                                                                               5000                                        Control - PGAA Spiked                                                                     0   210                                                                               290                                                                               370                                                                               300                                                                               --                                                                               -- -- --                                          p-Chloroaniline                                                                           0.12                                                                               62 300                                                                               370                                                                               210                                                                               28                                                                               300                                                                               380                                                                               310                                         p-Chloroaniline                                                                           0.47                                                                               <1 280                                                                               320                                                                               59 <1                                                                               290                                                                               350                                                                               270                                         p-Chloroaniline                                                                           0.99                                                                               <1 220                                                                               170                                                                               35 <1                                                                               290                                                                               310                                                                               270                                         Control - PGAA Spiked                                                                     0   220                                                                               270                                                                               330                                                                               310                                                                               --                                                                               -- -- --                                          1,5-Diaminonaphthalene                                                                    0.12                                                                               35 250                                                                               310                                                                               230                                                                               10                                                                               260                                                                               310                                                                               280                                         1,5-Diaminonaphthalene                                                                    0.47                                                                               <1 170                                                                               140                                                                               120                                                                               <1                                                                               270                                                                               310                                                                               140                                         1,5-Diaminonaphthalene                                                                    0.92                                                                               <1 200                                                                               160                                                                               120                                                                               <1                                                                               270                                                                               300                                                                               120                                         Control - PGAA Spiked                                                                     0   210                                                                               290                                                                               370                                                                               300                                                                               --                                                                               -- -- --                                          p-Methoxyaniline                                                                          0.13                                                                               71 300                                                                               380                                                                               200                                                                               32                                                                               300                                                                               370                                                                               310                                         p-Methoxyaniline                                                                          0.44                                                                               <1 290                                                                               340                                                                               66 <1                                                                               290                                                                               370                                                                               300                                         p-Methoxyaniline                                                                          0.93                                                                               <1 270                                                                               230                                                                               42 <1                                                                               300                                                                               370                                                                               300                                         Control - PGAA Spiked                                                                     0   190                                                                               380                                                                               430                                                                               300                                                                               --                                                                               -- -- --                                          o-Methylaniline                                                                           0.49                                                                               5  350                                                                               380                                                                               240                                                                               <1                                                                               340                                                                               370                                                                               290                                         o-Methylaniline                                                                           0.99                                                                               <1 300                                                                               290                                                                               170                                                                               <1                                                                               320                                                                               350                                                                               240                                         Control - PGAA Spiked                                                                     0   200                                                                               340                                                                               280                                                                               380                                                                               --                                                                               -- -- --                                          m-Methylaniline                                                                           0.10                                                                               59 340                                                                               280                                                                               270                                                                               30                                                                               300                                                                               250                                                                               370                                         m-Methylaniline                                                                           0.44                                                                               <1 310                                                                               250                                                                               140                                                                               <1                                                                               320                                                                               260                                                                               350                                         m-Methylaniline                                                                           0.91                                                                               <1 250                                                                               170                                                                               130                                                                               <1                                                                               300                                                                               220                                                                               370                                         Control - PGAA Spiked                                                                     0   97 310                                                                               280                                                                               340                                                                               --                                                                               -- -- --                                          p-Methylaniline                                                                           0.13                                                                               28 300                                                                               270                                                                               310                                                                               38                                                                               320                                                                               300                                                                               330                                         p-Methylaniline                                                                           0.62                                                                               <1 280                                                                               220                                                                               280                                                                               <1                                                                               300                                                                               270                                                                               320                                         p-Methylaniline                                                                           1.0 <1 260                                                                               180                                                                               300                                                                               <1                                                                               310                                                                               280                                                                               370                                         Control - PGAA Spiked                                                                     0   130                                                                               300                                                                               300                                                                               310                                                                               --                                                                               -- -- --                                          p-Nitroaniline                                                                            0.14                                                                               36 270                                                                               280                                                                               270                                                                                7                                                                               270                                                                               270                                                                               300                                         p-Nitroaniline                                                                            0.54                                                                               4  260                                                                               270                                                                               110                                                                               <1                                                                               280                                                                               250                                                                               160                                         p-Nitroaniline                                                                            1.1 4  250                                                                               260                                                                               62 <1                                                                               270                                                                               260                                                                               180                                         Control - PGAA Spiked                                                                     0   100                                                                               190                                                                               170                                                                               220                                                                               --                                                                               -- -- --                                          m-phenylenediamine                                                                        0.12                                                                               12 190                                                                               170                                                                               140                                                                                3                                                                               180                                                                               190                                                                               210                                         m-phenylenediamine                                                                        0.25                                                                               <1 170                                                                               150                                                                               48 <1                                                                               170                                                                               160                                                                               170                                         m-phenylenediamine                                                                        1.0 <1 72 22 13 <1                                                                                33                                                                                5 120                                         Control-CAA                                                                               0   79 210                                                                               210                                                                               7800                                                                              --                                                                               -- -- --                                          m-phenylenediamine                                                                        0.13                                                                               2  200                                                                               170                                                                               6100                                                                              --                                                                               -- -- --                                          m-phenylenediamine                                                                        0.33                                                                               <1 160                                                                               63 3700                                                                              --                                                                               -- -- --                                          m-phenylenediamine                                                                        0.67                                                                               <1 1  <1 3100                                                                              --                                                                               -- -- --                                          Control - PGAA Spiked                                                                     0   240                                                                               390                                                                               320                                                                               360                                                                               --                                                                               -- -- --                                          p-phenylenediamine                                                                        0.10                                                                               73 370                                                                               330                                                                               250                                                                               22                                                                               400                                                                               380                                                                               240                                         p-phenylenediamine                                                                        0.41                                                                               <1 320                                                                               190                                                                               28 <1                                                                               330                                                                               250                                                                               --                                          p-phenylenediamine                                                                        0.81                                                                               <1 250                                                                               90 -- <1                                                                               310                                                                               220                                                                               260                                         p-phenylenediamine                                                                        1.6 <1 160                                                                               35 -- <1                                                                               300                                                                               170                                                                               280                                         __________________________________________________________________________

Example 4

Screening Tests Exploring the Effect of Temperature on Aniline Addition to PGAA Spiked with Impurities (23°-25° C. and 60° C.)

In Example 4, the general procedure described previously for the screening tests was employed, except that (i) the amine employed was aniline, (ii) the test was conducted at either 23°-25° C. or 60° C., and (ii) the aliquots containing the amine were analyzed only once, after 30 min. The results are given in Table III, the data showing that the rate of reaction of the components with aniline after thirty minutes, in descending order was: acrolein>maleic anhydride>furfural>benzaldehyde. This reactivity trend is the same for 23°-25° C. and for 60° C. However, at 60° C., benzaldehyde, furfural, and maleic anhydride levels are higher than at 23°-25° C. with the same level of aniline. Thus, also to the advantage of less solids (previously discussed), higher temperatures, such as 50°-60° C., are preferred when adding Group A amine to CAA or an AA source.

                  TABLE III                                                        ______________________________________                                         Screening Tests Showing the Effect of Temperature on Removal of                Aldehydes by Aniline in Spiked PGAA                                                        Amine                                                                          Molar                                                              Additive    Ratio     A      B      F    M                                     ______________________________________                                         23-25° C.                                                               Control-Spiked PGAA                                                                        0         130    210    170  190                                   Aniline     0.22       14    190    170  87                                    Aniline     0.46      <1     150    120  25                                    Aniline     0.68      <1     140    91   20                                    Aniline     0.93      <1     120    79   32                                    Aniline     1.8       <1     88     50   36                                    60° C.                                                                  Control-Spiked PGAA                                                                        0         170    190    240  210                                   Aniline     0.38      <1     190    230  130                                   Aniline     0.46      <1     180    210  110                                   Aniline     0.68      <1     180    200  120                                   Aniline     0.92      <1     180    180  80                                    ______________________________________                                    

Example 5

Screening Tests Adding Hydrazine Hydrate and 2,4-Dinitro-phenylhydrazine to PGAA Spiked with Impurities

In Example 5, the general procedure described previously for the screening tests was employed, except that the amines employed were hydrazine and its derivatives. The results given in Table IV show that both hydrazine hydrate and 2,4-dinitrophenylhydrazine were effective as Group A amines; the latter amine was an outstanding Group B amine.

                                      TABLE IV                                     __________________________________________________________________________     Effect of Hydrazine Derivatives on Aldehyde Removal in Spiked PGAA                         Amine                                                                          Molar                                                                              After Thirty Minutes                                                                       After About Five Days                              Additive    Ratio                                                                              A  B  F  M  A  B  F  M                                         __________________________________________________________________________     Control - PGAA Spiked                                                                      0   210                                                                               320                                                                               310                                                                               320                                                                               -- -- -- --                                        Hydrazine Hydrate (55%)                                                                    0.54                                                                               11 300                                                                               280                                                                               90 <1 340                                                                               360                                                                               250                                       Hydrazine Hydrate (55%)                                                                    1.1 <1 260                                                                               220                                                                               27 <1 330                                                                               330                                                                               260                                       Control - PGAA Spiked                                                                      0   210                                                                               290                                                                               260                                                                               370                                                                               -- -- -- --                                        2,4-Dinitrophenylhydrazine                                                                 0.11                                                                               110                                                                               250                                                                               220                                                                               350                                                                               100                                                                               250                                                                               230                                                                               170                                       2,4-Dinitrophenylhydrazine                                                                 0.48                                                                               <1 58 78 280                                                                               1  35 55 160                                       2,4-Dinitrophenylhydrazine                                                                 0.96                                                                               <1 2  <1 64 <1 2  <1 53                                        __________________________________________________________________________

Example 6

Screening Tests Adding Alkylenepolyamine or (-Amino Acid to PGAA Spiked with Impurities

In Example 6, the general procedure described previously for the screening tests was employed, except that the amines employed were either alkylenepolyamines or α-amino acids. Here, glycine was added in aqueous solution. The results are given in Table V.

Table V screening test data show that diethylenetriamine, glycine, lysine, arginine and histidine were effective as Group A amines.

Example 7

Screening Tests Adding Structure IV ArylAmines to Spiked PGAA

In Example 7, the general procedure described previously for the screening tests was employed, except that the amines were either methyl anthranilate or 3-aminobenzoic acid. The tests were conducted at 48°-52° C. with a stock solution of CAA having the "control" analysis and results given in Table VI.

Table VI screening test data show that these substituted aryl amines were highly effective in reducing acrolein to below 10 ppm.

                                      TABLE V                                      __________________________________________________________________________     Effect of Alkylene Polyamines and α-Amino Acids on Aldehyde Removal      in Spiked PGAA                                                                            Amine                                                                          Molar                                                                              After Thirty Minutes                                                                       After About Five Days                               Additive   Ratio                                                                              A  B  F  M  A  B  F  M                                          __________________________________________________________________________     Alkylene Polyamines                                                            Control - PGAA Spiked                                                                     0   210                                                                               290                                                                               250                                                                               300                                                                               -- -- -- --                                         Diethylenetriamine                                                                        0.12                                                                                76                                                                               290                                                                               240                                                                               280                                                                               40 300                                                                               240                                                                               190                                        Diethylenetriamine                                                                        0.51                                                                                30                                                                               280                                                                               220                                                                               190                                                                               <1 300                                                                               220                                                                                62                                        α-Amino Acids                                                            Control - PGAA Spiked                                                                     0   210                                                                               290                                                                               250                                                                               300                                                                               -- -- -- --                                         Glycine (in H.sub.2 O)                                                                    0.37                                                                                92                                                                               300                                                                               250                                                                               220                                                                               25 280                                                                               200                                                                               270                                        Glycine (in H.sub.2 O)                                                                    0.74                                                                                88                                                                               300                                                                               250                                                                               200                                                                                3 240                                                                               160                                                                               250                                        Control - PGAA Spiked                                                                     0   200                                                                               290                                                                               360                                                                               310                                                                               -- -- -- --                                         DL-Lysine  0.12                                                                               120                                                                               280                                                                               330                                                                               310                                                                               22 250                                                                               280                                                                               250                                        DL-Lysine  0.44                                                                                47                                                                               270                                                                               330                                                                               290                                                                               <1 240                                                                               130                                                                               200                                        DL-Lysine  0.87                                                                                39                                                                               270                                                                               300                                                                               280                                                                               <1 180                                                                                10                                                                               160                                        Control - PGAA Spiked                                                                     0   240                                                                               270                                                                               330                                                                               320                                                                               -- -- -- --                                         Arginine   0.46                                                                                87                                                                               280                                                                               330                                                                               250                                                                               <1 240                                                                               160                                                                               230                                        Histidine  0.45                                                                                62                                                                               260                                                                               310                                                                               320                                                                               <1 260                                                                                10                                                                               310                                        __________________________________________________________________________

                                      TABLE VI                                     __________________________________________________________________________     Aldehyde/Maleic Anhydride Removal by Substituted Aryl Amines                              Amine                                                                          Molar                                                                              After Fifteen Minutes                                                                      After Five Hours                                    Additive   Ratio                                                                              A  B  F  M  A  B  F  M                                          __________________________________________________________________________     Control-Crude AA                                                                          0   119                                                                               147                                                                               198                                                                               5110                                                                              -- -- -- --                                         Methyl Anthranilate                                                                       0.72                                                                               <1 122                                                                               101                                                                               2745                                                                              <1 112                                                                                85                                                                               3139                                       Control-Crude AA                                                                          0   75 160                                                                               218                                                                               6261                                                                              -- -- -- --                                         3-Aminobenzoic Acid                                                                       0.55                                                                               <1 119                                                                                91                                                                               3203                                                                              <1 153                                                                               160                                                                               3802                                       __________________________________________________________________________

Example 8

Screening tests Adding Monoethanolamine to Spiked PGAA

In this Example, the general procedure described previously for the screening tests was employed, except that the amine employed was monoethanolamine (MEA). Aliquots containing MEA were stirred for 30 min. at room temperature, and analyzed; analysis also was made after a 5-day holding period. The results are given in Table VII. The screening test data show that monoethanolamine is highly effective in reducing acrolein to below 10 ppm after 5 days at r.t. and also in reducing measured maleic acid/anhydride levels to well below 70% of starting levels in the same period. While not as fast reacting as some Group A amines, above, MEA was effective at reducing acrolein in 30 minutes and very effective with extended holding time at room temperature. No insoluble solids were observed.

                                      TABLE VII                                    __________________________________________________________________________     Aldehyde/Maleic Anhydride Removal by MEA                                                  Amine                                                                          Molar                                                                              After Thirty Minutes                                                                       After About Five Days                               Additive   Ratio                                                                              A  B  F  M  A  B  F  M                                          __________________________________________________________________________     Control-PGAA Spiked                                                                       0   274                                                                               311                                                                               322                                                                               380                                                                               -- -- -- --                                         MEA        0.87                                                                               139                                                                               318                                                                               287                                                                               312                                                                               <1 337                                                                               361                                                                               168                                        MEA        1.90                                                                                58                                                                               319                                                                               241                                                                               232                                                                               <1 304                                                                               311                                                                               132                                        __________________________________________________________________________

Example 9

Preparation of PGAA where Group S Amine is MEA and Group P Amine is mPD

A two inch, fifteen tray Oldershaw column fitted with a steam heated, double pipe reboiler was used. A crude acrylic acid feed was treated by mixing 0.37 wt % of a Group S amine, monoethanolamine (MEA), with CAA at ambient temperature from 4 to 15 hours before it was added to the main feed reservoir of the distillation column. The pretreated crude acrylic acid feed was then heated by passing it through a separate heat exchanger. A Group P amine, m-phenylene diamine (mPD), feed was added to the preheated, pretreated CAA as the mixture was fed to the distillation pot. The contact time between the pretreated CAA and the mPD was ≦10 seconds before entering the pot. Two experiments were conducted.

Ex. 9A, in which mPD was added at 0.03-0.04 wt % on the pretreated crude acrylic acid; and

Ex. 9B, in which mPD was added at 0.08-0.09 wt % on the pretreated crude acrylic acid. Table VIII compares the analytical results of the CAA feed with that of the respective distillates.

                  TABLE VIII                                                       ______________________________________                                         Analytical Results, Examples 9A and 9B                                                           Column                                                                  Crude  Feed after                                                              Acrylic                                                                               MEA      Ex. 9A   Ex. 9B                                                Acid   Treatment                                                                               Distillate                                                                              Distillate                                 ______________________________________                                                      --        --                                                      Acrolein (ppm)                                                                              91        <1      <1     <1                                       Furfural (ppm)                                                                              1         145     1.4    <1                                       Benzaldehyde (ppm)                                                                          113       113     <1     <1                                       Maleic Acid/Anhydride                                                                       5636      3506    2      2                                        (ppm)                                                                          Mol MEA per mol of             0.98   0.98                                     Aldehyde and Maleic                                                            Mol mPD per mol of             0.05   0.13                                     Aldehyde and Maleic                                                            ______________________________________                                    

The data show that the described combination of MEA and mPD was highly effective in reducing impurities; the higher mol ratio of Ex. 9B also reduced furfural to <1 ppm. There was no observed solids formation in feed lines or reservoir, nor during distillation, thus demonstrating the effectiveness of the combination of the Group S amine and the Group P amine.

Comparative Example 6

CAA Distillation Where Group S Amine is Monoethanol Amine and with No Group P Amine Addition

This Example shows the results of distilling a Group S-treated CAA in the absence of a Group P amine. Treatment and distillation conditions similar to those of Example 9 were employed. CAA was treated with 0.37 wt % of a Group S amine, MEA, from 4 to 15 hours before it was added to the main feed reservoir of the distillation column, but no Group P amine was added to the pretreated feed or to the top of the distillation column before or during distillation. After running the distillation for ten hours, no polymer or other solids were observed in the column or pot, showing the effectiveness of the MEA in that function. However, the analytical results of Table IX show that residual impurities were not removed as effectively as when a Group P amine also was used (e.g. Ex. 9); furfural was higher than desired, although MEA effectively reduced acrolein to <1 ppm. and maleic anhydride to 2 ppm.

                  TABLE IX                                                         ______________________________________                                         Analytical Results, Comp. Ex. 6                                                               Crude    Comp.                                                                 Acrylic  Ex. 6.                                                                Acid     Distillate                                             ______________________________________                                         Acrolein (ppm)   77         <1                                                 Furfural (ppm)   193        8                                                  Benzaldehyde (ppm)                                                                              108        <1                                                 Maleic Anhydride (ppm)                                                                          5486       2                                                  Mol MEA per Mol Aldehyde    0.98                                               and Maleic                                                                     ______________________________________                                    

Comparative Example 7

CAA Treatment with No Group Amine

In this comparative run using no amine additivefrom any of the described Groups and under similar distillation conditions as used in Example 9, 18 ppm acrolein, 77 ppm of furfural, 3 ppm benzaldehyde and 46 ppm of maleic anhydride were obtained in the distilled CAA. This showed the effectiveness of both the Group S and Group P amines when used together, as in Example 9.

Example 10

Preparation of PGAA where Group S Amine is MEA and Group P Amine is Aniline

In these examples, MEA, a Group S amine, and aniline, a Group P amine, were sequentially used to produce PGAA. CAA was first treated with 0.37 wt % MEA at ambient temperature for 4 to 15 hours before it was added to the main feed reservoir of the distillation column. Aniline was fed to the pretreated crude acrylic acid just prior to distillation at 0.08 wt. % (Ex. 10A), at 0.30 wt. % (Ex. 10B) and at 0.60 wt. % (Ex. 10C) on the weight of pretreated CAA. The respective feeds were distilled yielding the analytical results summarized in Table X. After running a total of 23 hours, no polymer or solids were observed in the column or pot nor were any operational problems experienced. The results demonstrated the effectiveness of these amines in performing their respective functions.

                                      TABLE X                                      __________________________________________________________________________     Analytical Results, Examples 10 A-C                                                              Ex. 10A                                                                             Ex. 10A                                                                              Ex. 10B                                                                             Ex. 10C                                                   Crude                                                                               CAA Feed                                                                            Distillate                                                                           Distillate                                                                          Distillate                                                Acrylic                                                                             after MEA                                                                           0.08 wt %                                                                            0.30 wt %                                                                           0.6 wt %                                                  Acid Pretreat                                                                            Aniline                                                                              Aniline                                                                             Aniline                                      __________________________________________________________________________     Acrolein (ppm)                                                                              91   NM   1     <1   <1                                           Furfural (ppm)                                                                              181  145  6     1.8  1.5                                          Benzaldehyde (ppm)                                                                          113  99   <1    <1   <1                                           Maleic Anhydride (ppm)                                                                      5636 3506 2     NM   NM                                           Mol MEA per sum of Mol                                                                           0.98 0.98  0.98 0.98                                         Aldehyde/Maleic                                                                Mol Aniline per sum of Mol                                                                            0.14  0.52 1.0                                          Aldehyde/Maleic                                                                __________________________________________________________________________      Note:                                                                          "NM" was not monitored                                                   

Example 11

Screening Tests of Group S Amines with CAA

MEA and ethylene diamine were screened as representatives of a Group S amine. Screening tests were conducted by adding indicated levels of amine to aliquots of a stock solution of PGAA which had been spiked with the following impurities at 300 ppm each: acrolein (A), benzaldehyde (B), furfural (F), and maleic anhydride (M). The aliquots containing the amine were stirred for thirty minutes at 23°-25° C. and then immediately analyzed. After about five days at 23°-25° C., the aliquots containing amine were reanalyzed. Control samples were those spiked samples into which no amine was added. Table XI summarizes the analytical results, showing the effectiveness of these amines in reducing acrolein and, after 5 days, maleic anhydride. No substantial amount of insoluble solids was noted; only ethylene diamine gave more than 0.1 wt. % of solids, based on the weight of the mixture.

In similar tests of ortho- and meta-toluidine, at molar ratios of 0.9 to 1.0, acrolein was reduced to <1 ppm in 30 min., and maleic anhydride to <70 % of the starting level in the same time, without solids formation, demonstrating their effectiveness as Group S amines. (Para-toluidine, on the other hand, gave more than I wt. % solids in the screening test mixture and was not considered a satisfactory Group S amine.)

                                      TABLE XI                                     __________________________________________________________________________     Reactivity of Group S Amines with Aldehydes/Maleic Anhydride                              Amine                                                               Additive   Molar                                                                              After Thirty Minutes                                                                       After About Five Days                               (Impurity levels in ppm)                                                                  Ratio                                                                              A  B  F  M  A  B  F  M                                          __________________________________________________________________________     (From Table VII)                                                                          0   274                                                                               311                                                                               322                                                                               380                                                                               -- -- -- --                                         Control-PGAA Spiked                                                            MEA        0.87                                                                               139                                                                               318                                                                               287                                                                               312                                                                               <1 337                                                                               361                                                                               168                                        MEA        1.90                                                                                58                                                                               319                                                                               241                                                                               232                                                                               <1 304                                                                               311                                                                               132                                        Control-PGAA Spiked                                                                       0   140                                                                               330                                                                               372                                                                               405                                                                               -- -- -- --                                         Ethylene Diamine                                                                          0.86                                                                                4 317                                                                               274                                                                               403                                                                               <1 318                                                                               307                                                                               201                                        Ethylene Diamine                                                                          1.92                                                                                <1                                                                               315                                                                               200                                                                               369                                                                               <1 255                                                                               301                                                                               177                                        __________________________________________________________________________

Example 12

Screening Tests of Representative Group P Amines with CAA

Several representatives of Group P amines were screened for effectiveness in reducing impurities in CAA and spiked PGAA. Table II provides screening results for most of the defined Group P amines. Phenyl-hydrazine also was screened with CAA containing the impurities at the levels summarized in Table XII. Screening was at 48°-52° C. and the results after exposure times, as summarized in Table XII, demonstrated that phenyl-hydrazine had some utility in reducing furfural, particularly at a use level of 0.65 molar ratio. Acrolein levels were rapidly reduced to below 1 ppm.

The test with hydrazine in Example 2, which test was conducted without a Group B amine, demonstrated that hydrazine was nevertheless effective as a Group P amine, particularly in reducing benzaldehyde and furfural in the distillate to levels below 1 ppm.

                                      TABLE XII                                    __________________________________________________________________________     Reactivity of Phenyl-Hydrazine with Aldehydes/Maleic Anhydride                          Amine                                                                          Molar After Fifteen Minutes                                                                      After Five Hours                                    Additive Ratio A  B  F  M  A B   F  M                                          __________________________________________________________________________     Control-Crude AA                                                                        0     141                                                                               203                                                                               225                                                                               5447                                                                              --                                                                               --  -- --                                         Phenylhydrazine                                                                         0.21  <1 -- 184                                                                               5246                                                                              <1                                                                               --  163                                                                               4918                                       Control-Crude AA                                                                        0     135                                                                               195                                                                               219                                                                               5335                                                                              --                                                                               --  -- --                                         Phenylhydrazine                                                                         0.65  <1 --  47                                                                               4591                                                                              <1                                                                               --   19                                                                               4365                                       __________________________________________________________________________ 

We claim:
 1. A continuous process for producing a pure grade acrylic acid (PGAA) comprising the steps of:a) feeding to a distillation column a crude acrylic acid feed stream at a temperature of from 25° to 100° C., the feed stream comprising:i) a crude acrylic acid containing from 400 to 1100 ppm of aldehyde comprising acrolein,benzaldehyde and furfural, and ii) a minimum effective level in a range of from 0.1 to 2.0 molar ratio, based on total moles of aldehydes and maleic acid and maleic anhydride in the crude acrylic acid, of at least one of a Group A amine selected from the group consisting of:i) a primary arylamine of structure (IV), ##STR6## wherein X₃ is selected from the group consisting of --COOH and --COOR⁶, wherein R⁶ is selected from C₁ -C₆ alkyl, and ii) an alkanolamine of structure (V),

    HO--R.sup.7 --NH.sub.2                                     (V)

wherein R⁷ is selected from C₂ -C₆ alkylene; b) concurrently feeding to an upper portion of the distillation column an amine feed stream comprising from 0.01 to 1.0 molar ratio, based on total moles of aldehydes and maleic acid and maleic anhydride in the crude acrylic acid, of at least one of a Group B amine selected from the group consisting of o-, m-, p-phenylenediamine, 4-nitrophenyl-hydrazine, and 2,4-dinitrophenyl-hydrazine; and c) distilling the crude acrylic acid feed stream in the presence of the amine feed stream through the distillation column, thereby providing PGAA having a residual individual aldehyde content less than 10 ppm.
 2. The process of claim 1 wherein the alkanolamine is monoethanol amine.
 3. A continuous process for producing a pure grade acrylic acid (PGAA) comprising the steps of:a) feeding to a first distillation column an acrylic acid source feed stream at a temperature of from 25° to 100° C., the source feed stream comprising:i) an acrylic acid source containing from 400 to 1100 ppm of aldehyde comprising acrolein, benzaldehyde and furfural selected from the group consisting of an aqueous acrylic acid solution and an acrylic acid extract, the acrylic acid source optionally containing acetic acid; ii) from 0.1 to 2.0 molar ratio, based on total moles of aldehydes and maleic acid and maleic anhydride in the acrylic acid source, of at least one of a Group A amine selected from the group consisting of:i) a primary arylamine of structure (IV), ##STR7## wherein X₃ is selected from the group consisting of --COOH and --COOR⁶, wherein R⁶ is selected from C₁ -C₆ alkyl, and ii) an alkanolamine of structure (V),

    HO--R.sup.7 --NH.sub.2                                     (V)

wherein R⁷ is selected from C₂ -C₆ alkylene; b) dehydrating the acrylic acid source feed stream to provide a low acrolein crude acrylic acid having an acrolein content less than 10 ppm; c) optionally distilling off acetic acid from the low acrolein crude acrylic acid to provide a low acrolein crude acrylic acid having a reduced acetic acid level; d) subsequently feeding to a final distillation columni. the low acrolein crude acrylic acid, and ii. concurrently, to an upper portion of the final distillation column, an amine feed stream comprising from 0.01 to 1.0 molar ratio, based on total moles of aldehydes and maleic acid and maleic anhydride in the low acrolein crude acrylic acid, of at least one of a Group B amine selected from the group consisting of o-, m-, p-phenylenediamine, 4-nitrophenylhydrazine, and 2,4-dinitrophenyl-hydrazine; and e) distilling the low acrolein crude acrylic acid through the final distillation column, distilling off PGAA having a residual individual aldehyde content less than 10 ppm.
 4. A continuous process for producing a pure grade acrylic acid (PGAA) comprising the steps of:a) feeding to an acetic acid distillation column a feed stream at a temperature of from 25° to 100° C., the feed stream comprising:i) a crude acrylic acid containing greater than 2000 ppm acetic acid; ii) from 0.1 to 2.0 molar ratio, based on total moles of aldehydes and maleic acid and maleic anhydride in the high acetic acid-containing crude acrylic acid, of at least one of a Group A amine selected from the group consisting of:i) a primary arylamine of structure (IV), ##STR8## wherein X₃ is selected from the group consisting of --COOH and --COOR⁶, wherein R⁶ is selected from C₁ -C₆ alkyl, and ii) an alkanolamine of structure (V),

    HO--R.sup.7 --NH.sub.2                                     (V)

wherein R⁷ is selected from C₂ -C₆ alkylene; b) distilling off acetic acid from the feed stream to provide a low acrolein crude acrylic acid having an acetic acid content less than 2,000 ppm and an acrolein content less than 10 ppm; c) subsequently feeding to a final distillation column:i. the low acrolein crude acrylic acid, and ii. concurrently, to an upper portion of the final distillation column, an amine feed stream comprising from 0.01 to 1.0 molar ratio, based on total moles of aldehydes and maleic acid and maleic anhydride in the low acrolein crude acrylic acid, of at least one of a Group B amine selected from the group consisting of o-, m-, p-phenylenediamine, 4-nitrophenylhydrazine, and 2,4-dinitrophenyl-hydrazine; and d) distilling the low acrolein crude acrylic acid through the final distillation column, thereby providing PGAA having a residual individual aldehyde content less than 10 ppm.
 5. A process for producing a pure grade acrylic acid (PGAA) comprising the steps of:a) feeding to a crude acrylic acid feed stream containing from 400 to 1100 ppm of aldehyde comprising acrolein, benzaldehyde and furfural from 0.1 to 2.0 molar ratio, based on total moles of aldehydes and maleic acid and maleic anhydride in the crude acrylic acid feed stream, of at least one of a Group S amine selected from the group consisting of:i) an alkylenepolyamine of structure (II) ##STR9## wherein R³ is selected from H or a C₁ -C₆ alkyleneamine, and R⁴ is a C₁ -C₆ alkyleneamine; ii) ortho-toluidine, meta-toluidine; and iii) an alkanolamine of structure (V),

    HO--R.sup.7 --NH.sub.2                                     (V)

wherein R⁷ is selected from C₂ -C₆ alkylene; b) holding the crude acrylic acid feed stream and the fed Group S amine at a temperature of from 25° to 100° C. for a hold time of from 15 seconds to 150 hours to provide a treated feed stream; c) feeding the treated feed stream to a distillation column; d) concurrently feeding from 0.01 to 1.0 molar ratio, based on total moles of aldehydes and maleic acid and maleic anhydride in the crude acrylic acid, of at least one of a Group P amine selected from the group consisting of o-, m-, p-phenylenediamine, 4-nitrophenyl-hydrazine, 2,4-dinitrophenyl-hydrazine, phenyl-hydrazine, hydrazine, and aniline, to the treated feed stream fed to the distillation column, optionally to an upper portion of the distillation column; and e) distilling the treated feed stream, in the presence of the Group P amine, through the distillation column, thereby providing PGAA having a residual individual aldehyde content less than 10 ppm.
 6. A process for producing a pure grade acrylic acid (PGAA) comprising the steps of:a) feeding to an acetic acid distillation column a feed stream at a temperature of from 25° to 100° C., the feed stream comprising:i) a crude acrylic acid containing greater than 2000 ppm acetic; ii) from 0.1 to 2.0 molar ratio, based on total moles of aldehydes and maleic acid and maleic anhydride in the high acetic acid-containing crude acrylic acid, of at least one of a Group S amine selected from the group consisting of:i) an alkylenepolyamine of structure (II) ##STR10## wherein R³ is selected from H or a C₁ -C₆ alkyleneamine, and R⁴ is a C₁ -C₆ alkyleneamine; ii) ortho-toluidine, meta-toluidine; and iii) an alkanolamine of structure (V),

    HO--R.sup.7 --NH.sub.2                                     (V)

wherein R⁷ is selected from C₂ -C₆ alkylene, to provide an amine treated feed stream; b) distilling off acetic acid from the amine-treated feed stream to provide a low acetic acid, amine-treated crude acrylic acid having an acetic acid content less than 2,000 ppm; c) feeding the the low acetic acid, amine-treated crude acrylic acid to a distillation column; d) concurrently feeding from 0.01 to 1.0 molar ratio, based on total moles of aldehydes and maleic acid and maleic anhydride in the crude acrylic acid, of at least one of a Group P amine selected from the group consisting of o-, m-, p-phenylenediamine, 4-nitrophenyl-hydrazine, 2,4-dinitrophenyl-hydrazine, phenyl-hydrazine, hydrazine, and aniline, to the low acetic acid, amine-treated crude acrylic acid fed to the distillation column, optionally to an upper portion of the distillation column; and e) distilling the low acetic acid, amine-treated crude acrylic acid, in the presence of the Group P amine, through the distillation column, thereby providing PGAA having a residual individual aldehyde content less than 10 ppm. 